The high efficiency of switching power converters such as a flyback converter has led to their virtual universal adaption as the battery charger for mobile devices. Since a flyback converter converts AC household voltage, fault conditions such as too much voltage across the power switch (designated as an over-voltage fault) may be potentially dangerous. It is thus conventional for a flyback converter controller to monitor for various fault conditions. Should the controller detect the presence of a fault condition, it stops cycling the power switch and enters a re-startup period. Upon the conclusion of the re-startup period, the controller may again resume normal operation.
One type of fault relates to the auxiliary winding. In primary-only feedback techniques, the controller modulates the cycling of the power switch cycling responsive to a feedback voltage obtained from the auxiliary winding to regulate the output voltage. If the controller's connection to the auxiliary winding has an open-circuit fault, the feedback voltage will be too low such that the controller will drive the output voltage out of regulation and perhaps dangerously high. It is thus conventional for the controller to monitor an auxiliary winding T2 through a dedicated auxiliary winding terminal (V_Aux) as shown in FIG. 1 for a primary-side controller U1 in a conventional flyback converter 100. A rectified input voltage V_IN as smoothed by an input capacitor C1 drives a magnetizing current through a primary winding of a transformer T1 when controller U1 switches on a power switch transistor S1 through a drive voltage DRV applied to the gate of power switch transistor S1. Controller U1 monitors the primary winding current by sensing a voltage across a sense resistor Rs through an Isense terminal to switch power switch transistor S1 off when a desired peak primary winding current is satisfied. Controller U1 also has a ground (GND) terminal for coupling to ground.
Referring now to FIG. 2 in conjunction with FIG. 1, the primary winding current (I_prim) ramps up to the desired maximum value while power switch transistor S1 is on. During this on time, an output diode D1 prevents a secondary current (I_sec) from flowing. In synchronous rectification embodiments, output diode D1 would be replaced by a synchronous rectification switch. When controller U1 switches off power switch transistor S1, the primary winding current ceases whereas the second current spikes to a maximum value and then begins ramping down to zero. The point at which the secondary current ramps to zero is known as the transformer reset time and represents the ideal time for controller U1 to sense the output voltage V_OUT by sensing the reflected voltage on the auxiliary winding. To do so, controller U1 senses a feedback voltage through a feedback voltage (VFB) terminal connected to a voltage divider formed by a pair of resistors Ra and Rb that are in series with the auxiliary winding. The auxiliary winding voltage is also rectified through a diode D1 and stored on a capacitor C3 to produce a power supply voltage Vcc received by controller U1 on a Vcc terminal.
While the secondary current flows, a voltage Vd-s is impressed across the drain and source terminals for power switch transistor S1. The Vd-s voltage equals a sum of the input voltage V_IN and N*V_OUT, where N is the turn ratio for transformer T1. Controller U1 monitors the input voltage V_IN through a Vin_Sense terminal and can thus indirectly measure Vd-s since the V_OUT and the turn ratio N are both known. In this fashion, controller U1 can monitor whether Vd-s is excessively high so as to violate a Vd-s (MAX) threshold as shown in FIG. 2. Similarly, controller U1 monitors the Vaux terminal voltage to determine whether a Vaux switching threshold is satisfied. Should the Vaux terminal be properly coupled to the auxiliary winding without an open circuit fault, the Vaux terminal voltage drops to a negative voltage while power switch transistor S1 is on and then rises over the positive Vaux switching threshold in response to power switch transistor S1 switching off. The Vaux terminal voltage then drops to zero after the secondary winding current drops to zero to complete its behavior during a cycle of power switch transistor S1. Should the Vaux switching threshold not be crossed, controller U1 determines that the auxiliary winding is open-circuited to controller U1.
Although it is important to sense whether the input voltage is too high and whether the auxiliary winding has an open-circuit fault, detection of both fault conditions requires two separate terminal on the controller. Each required terminal for an integrated circuit increases manufacturing cost and complexity. Accordingly, there is a need in the art for a flyback controller that can monitor for over-voltage faults on the power switch and also monitor for open-circuit faults for the auxiliary winding without requiring two dedicated terminals.